Remove Geo system hooks

What does this MR do?

In 9.4, we deprecated system hooks for Geo.

In 10.0, we should kill them for good. Here's what we need to do to remove them:

• Remove Geo API receive hook handling
• Change the GeoNode model's repair functionality (update_dependents_attributes)
• Create a migration to remove the system hook
• Add some sort of diagnostic/warning that SSH key lookups via DB is the only supported way

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

gitlab-org/gitlab-ee#3110